This invention relates generally to the technology of magnetic separation, and more specifically to method and apparatus for removal of magnetically more susceptible minute particles, often present in minor concentration as coloring impurities, from aqueous slurries of minute mineral particles--such as obtained by dispersing clay, e.g., a crude kaolin clay, in water.
The iron content of commercial deposits of kaolin clay is generally on the order of from approximately 0.2% to 2%. Even recent publications indicate a continuing dispute as to whether the iron contaminants are in discrete form or in a combined form within a kaolin lattice structure. While the form of this iron in clay has not been definitiely established, recent evidence indicates that a portion is concentrated in or associated with nonkaolin contaminants such as titanium oxides, etc. Whatever its form, iron contamination reduces brightness in clay and the degree of discoloration of the clay generally increases with the amount of iron present.
In the foregoing connection, it has been known for some time that magnetically attractable contaminants can to a degree be removed from aqueous slurries of the aforementioned clays by imposition on the slurry of a high intensity magnetic field gradient. The forces produced upon the particles by the magnetic field gradient, effect differential movements of mineral grains through the field, in accordance with the magnetic permeability of the minerals, their size, mass, etc. The difficulties of utilizing magnetic separation are compounded in the present environment by the fact that the contaminants of greatest interest are are relatively low attractability. The primary magnetic discolorant found in Middle Georgia clays, for example, is iron-stained anatase (TiO.sub.2). This impurity is very small in size and only very weakly magnetic. Indeed by some early views contaminants of this general type were considered to be non-magnetic. For example, see A. F. Taggart, Handbook of Mineral Dressing, p. 13-02 (1960), which shows on a scale of 100.00 taking iron as a standard, that the relative attractability of TiO.sub.2 is 0.37.
In the copending patent applications of Joseph Iannicelli, Ser. No. 19,169, filed Mar. 13, 1970; Ser. No. 309,839, filed Nov. 27, 1972; and Ser. No. 340,411, filed Mar. 12, 1973 which applications are assigned to the assignee of the instant application, there are disclosed method and apparatus, which in comparison to the prior art, are outstandingly effective in achieving magnetic separation of the low susceptibility impurities referred to. In accordance with the disclosure of said applications, a container adapted to have the slurry passed therethrough is filled with magnetizable elements (preferably steel wool), constituting a flux conductive matrix, which matrix serves both for diverting the slurry flow into multitudinous courses, and for concentrating magnetic flux at myriad locations therein, so as to collect the weakly susceptible particles from the slurry. This container or canister, as it is referred to therein, is preferably of non-magnetic construction and disposed end-wise or axially between confronting surfaces of ferromagnetic pole members, between which a magnetic field having a high intensity is produced throughout the matrix. Preferably the said canister is generally cylindrical in form, and is oriented between the pole members with its axis vertical, its ends being adjacent to and covered by the pole members. In the first two of the cited Iannicelli applications, the flow of slurry through the canister and matrix is in the same general direction (i.e., axial) as the high intensity magnetic field. In the last listed of the said applications, it is disclosed that certain important advantages accrue from flowing the slurry through the canister in such manner that the predominant direction of flow through the matrix is radial, i.e., from the outside diameter (O.D.) thereof toward the axis, or from the axis toward the O.D.
The slurry, as taught in the said Iannicelli applications, is passed through the container at a rate sufficient to prevent sedimentation, yet slow enough to enable the collection and retention of weakly magnetic particles from the flow onto the matrix elements. The magnetic field which is applied during such collection, is taught in the said applications to have an intensity of at least 7,000 gauss, and preferably has a mean value in the matrix of 8,500 gauss or higher. At such field strengths magnetic saturation of the matrix occurs. After a sufficient quantity of magnetics are collected, slurry flow is discontinued, and with the field cut off the matrix is rinsed and flushed.
While the Iannicelli apparatus and method above-described hae indeed been found highly effective for the desired purposes, it has nevertheless been observed in practice that apparatus and methods yielding a given set of results in a first environment would provide unanticipated (and in some instances, unacceptable) results in a differing environment. For example, a specific canister and matrix operating upon slurries having different particle characteristics and different viscosities, might display unexpectedly poor results, even when the same field intensities and flow conditions were utilized. In consequence operation and design of systems of the described type, have been based on trial and error, and on such guidance as could be provided by application of the intuitive sense. Such approach, however, has not enabled development of optimized systems, nor has it established correct modes of operation where trade-offs are required in the system operation.
For example, up to the present time, it has not been appreciated what options were available were one desirous in systems of the foregoing type of reducing retention time for the slurry in the separation (thereby increasing production rates), without sacrificing brightness in the resultant product. In the Bulletin of the American Physics Society Vol. 16 (1971) at page 350, for example, C. P. Bean reports an equation pertinent to removal of suspended particles in a fluid passed through a magnetic field, without however teaching any practical applications or limitations for the mathematical concepts mentioned.
In my copending application, Ser. No. 495,712, filed Aug. 8, 1974, and entitled METHOD AND APPARATUS FOR MAGNETIC BENEFICIATION OF PARTICLE DISPERSIONS, I disclose my finding that performance of separating systems of the type disclosed in the cited Iannicelli applications, by which it is meant reduction of discoloring magnetic contaminants and brightness improvement in the remaining product, is given in terms of a parameter p. This parameter, henceforth referred to as the "Separation parameter," is given by the expression: EQU p=Q/.eta.(d/D).sup.2 MH.tau.X(1-X) (1)
where Q is the magnetic susceptibility and d the means particle diameter of the attractable contaminant particles, .eta. is the viscosity of the fluid slurry including the particles, M is the magnetization and D the mean diameter of the filaments of the separation matrix, X is the fraction of the canister volume occupied by the matrix, H is the intensity of the applied magnetic field, and .tau. is the retention time in the said field. The parameter p is related to the factor C.sub.o /C, representing the ratio of contaminant particles (C) entering the separation system to the particles (C.sub.o) leaving the system, by the expression: EQU C.sub.o /C=e.sup.-.alpha.p ( 2)
where .alpha. is a numerical coefficient characteristic of the system. By determinately selecting among the controllable variables of the separation system a desired C.sub.o /C ratio is yielded. In a typical instance for example, the factors Q, .eta., M and d are presented as essentially fixed quantities, so that a desired C.sub.o /C ratio is provided by selection among the controllable factors D, H, .tau., and X.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for magnetic separation of low magnetic susceptibility discolorant particles from aqueous clay slurries, whereby production rate may be controlled or optimized for a given brightness increase in the clay.
It is another object of the present invention, to provide method and apparatus for magnetic separation of low magnetic susceptibility particles from aqueous slurries of said particles with comparatively larger number of non-magnetic particles, according to which determinative trade-offs may be provided among the controllable variables in the separation system, thereby tailoring the system performance characteristics to the materials being treated, to desired production rates, available magnetic field intensities, and so forth.